Amplifier



Jan. 28, 1941. PQSTHUMUS 2,230,122

AMPLIFIER Fiied Oct. 9, 1957 AWL/H5123 WE mpur 54- AMPLIFIER 4. $7Ann/P150155 5E INVENTOR x4445 figs yum/s .1 BY KZ ATTORNEY Patented Jan.28, 1941 UNITED STATES AMPLIFIER- Klaas Posthumus, Eindhoven,Netherlands, assignor, by mesne assignments, to Radio Corporation ofAmerica, New York, N.'Y., a corporation of Delaware Application October9, 1937, Serial No. 168,155 In the Netherlands October 22, 1936 12Claims.

The invention relates to an amplifying system for modulated highfrequency oscillations, which is particularly suitable for the finalstage of a transmitter.

It is known to utilize for the amplification of modulated high frequencyoscillations an amplitying tube wherein the grid bias has such a valuethat the tube passes current only during one half wave of thealternating voltage to be amplified. Such an amplifier is known underthe name of Class B amplifier" and with full load it has an efiiciencyof if the amplitude of the alternating voltage set up in the outputcircuit is assumed to be equal to the direct anode voltage. In practice,however, a maximum anode alternating voltage is allowed which is equalto 0.8 to 0.9 times the direct, anode voltage with the result that themaximum efficiency of a class B amplifier is in practice not higher than0.8 to 0.9 times When such an amplifier is employed for theamplification of modulated high frequency oscillations this efficiencyof 67% only occurs with 100% modulation of the carrier wave whereas theefficiency of the amplification of the unmodulated carrier wave cannotbe higher than about 33%. For broadcast transmitters the averagepercentage of modulation over an entire day is very small while theinstantaneousvalue of the percentage of modulation seldom surpasses 30%,or in other words the efficiency of a class B amplifier is on theaverage not much higher than 33%.

In describing my invention in detail reference will be made to theattached drawing wherein,

Figures 1 and 2 illustrate fundamental circuit arrangements utilizingthe principle of my invention, while Figures 3 and 4 illustratemodifications of the arrangements of Figures 1 and 2, and,

Figure 5 illustrates details of the arrangement of Figure 4.

In order to obtaina higher efficiency it has previously been proposed toutilize the system shown in Figure 1; In this system I denotes a class Bamplifier which is loaded by an impedance which behaves for thefrequency of the carrier oscillation as an ohmic resistance of the value2R. The output circuit of the amplifier I comprises the output terminals5 and 6 of a network I while the input terminals 8 and 9 are connectedto an amplifier 2 whose bias voltage is chosen in such manner that thisamplifier only passes current if the amplitude of the oscillations to beamplified surpasses that of the carrier oscillation. The oscillations tobe amplified are supplied to the amplifiers I and 2 with a mutual phasedisplacement of The network I is constructed to have the reversibleproperty that the impedance occurring across the output terminals 5 and6 is inversely proportional to the impedance connected between the inputterminals 8 and 9. The product of said impedances is R0 where Ro=R andis hereinafter referred to as surge impedance.

So long as the amplifier 2 is not conductive, or in other words theamplitude of the oscillations to be amplified is smaller than thecarrier wave amplitude, the impedance occurring across the terminals 5and 6 will be zero and the amplifier I will be loaded by the resistance2R. If the amplitude of the oscillations to be amplified surpasses thecarrier wave amplitude the amplifier 2 becomes conductive during thatportion oi'the oscillations to be amplified wherein the instantaneousvalue of the oscillations surpasses the carrier wave amplitude whileacross the termi nals 5 and B a negative resistance occurs which in thecase of maximum conductivity of the amplifier 2, which occurs at thepeak voltage of a modulated carrier oscillation, is equal to The load ofthe amplifier l consequently depends on the instantaneous value of theoscillations to be amplified in such manner that the load amounts to 2Ras long as the amplitude of the oscillations to be amplified is smallerthan the amplitude of the unmodulated carrier oscillation, whereas theload is reduced from 2R to R if the instantaneous value of theoscillations to be amplified increases to the peak voltage of the 100%modulated carrier wave, i. e., to a value which is equal to twice thecarrier wave amplitude. The load of the amplifier 2 is determined by theimpedance connected between the terminals 5 and 6, which impedance isdetermined by the conductivity of the amplifier I. The circuitarrangement of the amplifier I is so chosen that if the instantaneousvalue of the oscillations to be amplified increases from the carrierwave amplitude to double this value, the negative resistance presentedby the amplifier I decreases from 2R to R and consequently theresistance present between. the terminals 5 and 6 increases from O to R.The

the amplifier 2 is blocked and the amplifier I lis', I

loaded by a resistance 2R which is so chosen that the highest allowableanode alternating voltage of the amplifying tubes is attained when 'theamplitude of the oscillations to be amplified is equal to theunmodulated carrierwave ampli- I tude. The efiiciency of the amplifierI, which increases with an increasing amplitude-of; the; oscillations tobe amplified, has consequent1y,at-;

tained with unmodulated carrier wave ampli: tude its maximum value of67%. When the amplitude of the oscillations to 'be amplified surpassesthat of the unmodulatedbarrier, Wave, the amplitude of the'alternatingvoltage setup in the output circuit of the amplifier-I can no longerincrease and, but for the presence of the amplifier 2, the output energyofthe zamplifier would increase no longer. At this moment-however, theamplifier 2 becomes conductive with the result that the load resistanceof the amplifier I decreases and an increase of the output energy of theamplifier I can be obtained by an increase of the anode current-ottheamplifying tubes without -any increase =of th e anode alter natingVoltage, which has already attained its maximum value. At the maximumvoltage of the 100% modulated carrier oscillation, the-load of theamplifier I is-equal to Rwhile the output energy ofthe amplifier I istwice as large as in the amplification of an'unmodulated carrieroscillation. At the peak value of the voltage of the 100% modulatedcarrier oscillation the load of the amplifier 2 is also equal to R whilethe energy supplied by this amplifier is equal to the energy'deliveredby the amplifier I. The total output energy of the amplifiers I and 2 isconsequently'equal to four times the output energy of the amplifier Iduring the amplification-of an unmodulated carrier oscillation.

When a 100% modulated carrier oscillation is amplified, theefiicien'oy-of the amplifier 'I is slightly smaller than with anunmodulated carrier oscillation, for example 50%, while the efficiencyof the amplifier 2, which acts as a class C amplifier, is higher, forexample With modulation the efiiciency of the total system isconsequently approximately as high as withunmodulated carrier wave. Withrespect to the .usual class B amplifiers the system describedconsequently offers the advantage that the average efiiciency over anentire day is considerably,

higher. In contra'distinction to that which is the case with class Bamplifiers this eificiency' is, however, always lower than theefiiciency obtained when an unmodulated carrier Wave is amplified.

FigureZ represents a system which is completely dual to the systemdescribed and which functions in a similar. way. With this system the,amplifier I is connected to a load resistance viaa network "I which hasthe same property as the network 1 in Figure 1. connected to the ends ofthis resistance and possesses a bias voltage such that current is onlyThe amplifier 2 is passed when the amplitude of the oscillations to beamplified surpasses the carrier wave amplitude. The surge impedance R0of the network I amounts to R.

During the amplification of the unmodulated carrier wave the amplifier 2is unoperative while the amplifier -I,,; is loaded by an impedance Ifthe instantaneous value of the oscillations to be amplifiediincreases'to above the carrier wave amplitude; theamplifier 2 becomes conductiveso that inpa'rallel with the load resistance carrier oscillation to Rwith the result-that theamplifier "I is loaded 'by a resist'ance Whenthe instantaneous value of the oscillations to be amplified "increasesfrom the carrier wave amplitude up to the maximum voltage of the 100%modulated carrier oscillation, the load of the amplifier 2 decreasesfrom to R. A

The operation 'of this system further corresponds completely to that ofthe system according to Figure 1, while a more detailed discussion ofthe system appears in the publication by Doher-ty at page 1153 of Y theSeptember, 1936, I. R. E.

The invention concerns an improvement in a system which operates on theprinciple of the systems shown in Figures 1 and 2.

According to the invention, for the amplification of modulated highfrequency oscillationsuse is made of four amplifiers'of which oneamplifier (I) passes current, with any amplitude of the alternatingvoltage supplied, during one half wave whereas the other amplifiers onlypass current if the amplitude is located above a predetermined thresholdValue which is for one of these amplifiers (2) lower and for the twoother amplifiers 3 and 4) mutually equal and larger than the carrierwave amplitude while a load impedanceis connected in series or inparallel with animpedance to which "a voltage is supplied by the firstmentioned amplifier (I) or the second amplifier (2). This series orparallel connection'is connected to the output terminals of a networkhaving connected to its input terminals an impedance across which thesecond amplifier (2) or the first amplifier (I) set up a voltage whileeach of the said impedances is connected in series with the outputimpedance of a network to the input terminals of which are connected thethird and the fourth amplifier (3 and 4) respectively. All thesenetworks possess the property that the input impedance is inverselyproportional to the impedance connected between the output terminalswhile the surge impedance of the latter two networks is equal to halfthe surge impedance of the first mentioned network, which surgeimpedance is equal respectively to half 'or double the load impedancewhile the Oscillations to be amplified are supplied to the grid of thefour amplifiers with a phase such that the voltages supplied by thesetubes to the load impedance are mutually in phase.

The invention willbe explained more fully with with the output ofamplifiers 3 and 4. At the reference to Figures 3 and 4 of the drawingwhich represent two embodiments thereof.

Figure 3 represents a system which may be considered as an extension ofthe system according to Figure 1. In this figure, I denotes a class Bamplifier which is loaded by a resistance 2R. In the output circuit ofthe amplifier are located the terminals 5 and 6 of a network I to theinput terminals of which is connected an amplifier 2 whose bias voltageis chosen in such manner that this amplifier only passes current if theamplitude of the oscillations to be amplified surpasses a predeterminedvalue which is smaller than the carrier wave amplitude. In the circuitwhich comprises the amplifier I, the load resistance 2B and theterminals 5 and B, is included the output impedance of a network IIwhich has connected to its input terminals an amplifier 4 of which thebias voltage is so chosen that this amplifier only passes current if theamplitude of the oscillations to be amplified surpasses a predeterminedvalue which is greater than the carrier wave amplitude. In the circuitwhich comprises the amplifier 2 and the terminals 8 and 9 of'the networkI is included the output impedance of a network II) which has connectedto its input terminals an amplifier 3 whose bias voltage is equal tothat of the amplifier 4. The networks I, II] and II possess the propertythat the input impedance is inversely proportional to the impedanceconnected between the output terminals, the said property beingreversible. The surge impedance R0 of the network "I amounts to R. whilethose of the networks I0 and II are Since each of the networks I, I0 andII brings about a phase 'displacement of 90 the high frequencyalternating voltages supplied to the input circuits of the amplifiers 2,3, and 4 are displaced in phase by 90, 180 and 90 respectively withrespect to the voltage supplied to the amplifier I.

If an unmodulated carrier oscillationis supplied to the amplifiers, theamplifier I lsoperative during the full wave and the amplifier 2 onlyduring part of the wave. During that part of the wave in which theamplifier 2 is not conductive, the amplifier I is loaded by a resistance2R. At the moment when the amplifier 2 becomes conductive, theinstantaneous value of the voltage set up in the output circuitof theamplifier I attains its highest admissible value so that the amplifier Ifunctions at this moment with the maximum efficiency. During the waveportion wherein the amplifier 2 is conductive, the load of the amplifierI is smaller than 2R owing to which an increase of the anode current ofthe amplifying tubes is possible without any increase of the anodealternating voltage.

At the moment when the amplifiers 3 and 4 become conductive, the anodealternating voltage of the tube of the amplifier 2 has attained itshighest value permissible and the load of the amplifier I is equal to Rwhile furthermore the load of the amplifier 2 amounts to R. At thismoment the amount of energy furnished by each of the two amplifiers Iand 2 is equally large. If the instantaneous value of the oscillationsto be amplified surpasses the threshold value of the amplifiers 3 and 4,the load on the amplifiers I and 2 decreases due to the fact'that anegative resistance occurs between the output terminals of the networkII or II] respectively. The negative resistance increases in accordancemaximum voltage of the'100% modulated carrier wavethis negativeresistance has attained its maximum value /ZR) and the amplifiers 3 and,4 supply maximum energy while the load of the amplifiers I and .2 hasdecreased from I the value It to the value /2R. Owing to the increase ofthe anode currents of the tubes of the amplifiers I and 2, the outputenergy of these amplifiers has increased to double the value at themoment when the threshold value of the amplifiers 3 and 4 is beingsurpassed. At the maximum voltage of a modulated carrier oscillation theamplifiers 3 and 4 are each loaded with a resistance it since the surgeimpedance R0 of the networks ID and II amounts to 2 At this moment eachof the amplifiers I, 2, 3 and 4 delivers an equally large amount ofenergy and the efficiency of the whole of the installation is equal tothe maximum efiiciency of a class B amplifier which furnishes the sameoutput energy.

A particular advantage of the system according to the invention residesin that at the most usual values of the percentage of modulation, whichare located between 0 and 30% the amplifiers I and 2 are continuously inoperation, owing to which discontinuities due to the tube 2 beingswitched on and off are avoided. A further advantage consists in that atthe most usual values of the percentage of modulation there occurs alinear relation between the amplitudes of the oscillations to beamplified and the current passing through the load resistance. Besides,in the system according to the invention the average efficiency over an.entire day is substantially equal to the efiiciency with unmodulatedcarrier oscillation, said efficiency being approximately 67%.

Figure 4 represents a system according to the invention which iscompletely dual to the system according to Figure 3 and functions in. asimilar manner. In this system the amplifier I is connected to a loadimpedance via a network I which possesses the same property as thenetwork I in the system of Figure 3. The amplifier 2 is connected to theends of this impedance. The surge impedance of the filter I amounts toR. In the circuit which co'mprises the amplifier I and the terminals 8and 9 of the network I is included the output impedance of the networkII whose surge impedance amounts to and to the input terminals of whichis connected the amplifier 4. In the circuit which comprises theamplifier 2 and the load resistance is included the output impedance ofa network III whose surge impedance also amounts to .the carrier wave.prise each a discharge tube 33 and 34, respectively, to which theoscillations to be amplified are and whose input terminals haveconnected'to them .the amplifiers 3. 'The bias voltages and the mutualphase displacements of the oscillations supplied to the input circuitsof the amplifiers l, 2, 3 and 4 are the same as in the system "accordingto Figure 3. resistances of the different amplifiers as a func- Thevariation of the load tion of the modulation is also completely equal tothat in the system according to Figure 3.

Figure 5 represents one mode of execution of the system according toFigure 3. In this systom the amplifier I is constituted by a'dischargetube 2| to the control grid of which are supplied the oscillations to beamplified and in the anode circuit of which is included a parallelcircuit 3| which is tuned to the carrier wave frequency and which iscoupled with an inductance 4| which forms jointly with a condenser 5| animpedance across which a voltage is. set up by the amplifier I. Theamplifier 2 comprises a discharge tube 22 to which the oscillations tobe amplified are supplied with a phase displacement of 90 and whoseanode circuit comprises a parallel circuit '32 which is coupled with aseries circuitconsist- .ing or" an inductance 42 and a condenser 52.-The filter 7 consisted two equal condensers 5'! and 51 and aninductance coil 41, whilethe two condensers are connected in seriesbetween the terminals 5 and 8 and the junction point of the condensersis connected via the inductance 41 to the terminals 6 and 9. Thenetworks In and II consist each of two coupled inductances 43, 43 and44, 44' respectively of which the inductances 43 and 44 respectively areconnected .in-series with condensers 53 and 54 respectively .and theother inductances 43' and 44 respectively are connected in series withcondensers 53 and 54 respectively between the output terminals, said twoseries connections being tuned to The amplifiers 3 and 4 comsuppliedwith a phase displacement of 180 and 9Q? respectivelyand in the anodecircuit of which may each comprise any desired number of amplifyingtubes connected in cascade while one or more stages of this cascade mayconsist of two push-pull connected amplifying tubes.

If either of the amplifiers 3 and 4 should be rendered ineffective thesystem will operate with only the remaining amplifiers. Appropriateadjustment of the amounts of power fed into the amplifiers and of thebias of the amplifiers should be made in the manner suggested for theheretofore known systems in the above mentioned IRE publication so thattheiload will be divided in an equitable manner.

I claim:

1. A system for amplifying modulated high frequency oscillations whichis particularly suitable for the final stage of a transmitter and whichcomprises four amplifiers each having an input and an output circuit,the first said amplifier being adjusted to pass current with anyamplitude of alternating voltages applied thereto during one i'h'alf'wave, whereas the other amplifiers are ad-,

justed to pass'current only if the amplitude'is'located above apredetermined threshold value which is 'forthe second of theseamplifiers lower and-for the third and fourth amplifiers mutuallyequaland' larger than the carrier wave amplitude, a plurality ofnetworks having input and output terminals and each having the propertythat the input impedance is inversely proportional to the impedanceconnected between the output terminals, a load impedance connectedinseries with theoutputs of thefirst and second of said networks and animpedance to which avoltage is supplied by said first amplifier,thasurge impedance of said first mentioned networkxbeing equal to halfthe loadimpedanceand having connected to its input'terminals a seriesconnection of an impedance across which a voltage is set up bysaid-second amplifier and the output of a third network having its inputterminals connected to said third amplifier, the input terminals of saidsecond network being connected to the fourth amplifier, the surgeimpede'nce of saidsecond and third networks being half that of the firstnetwork, said oscillations to be amplified beingi'sup-' plied to'theinputs of the four amplifiers in such phase relationship that thevoltages supplied by the amplifiers to the load impedance are mutuallyin phase.

2. A system as claimed in claim 1, wherein the said impedances acrosswhich voltages are set up by the first and the second amplifier consisteach of the series connection of an inductance and a condenser, saidinductance being coupled with a parallel resonant circuit included inthe output circuit of the amplifier and which is tuned to the carrierwave.

'3. A system as claimed in claim 1, wherein the first mentioned networkconsists of two condensers and an inductance, said condensers beingconnected in series between one of the input terminals and one of theoutput terminals whereas the inductance is located between the junctionpoint of the condensers and the two other terminals.

4. A system as claimed in claim 1, wherein the networks to the inputterminals of which are connected the third and the fourth amplifierrespectively comprise each two coupled inductances of which one isconnected in series with a condenser between the input terminals and theother in series with a condenser between the output terminals, boththese series connections 'tionship, said circuits including networkshaving inputs and outputs, the impedance at the inputs of said networksbeing inversely proportional to the impedance at the output thereof. 7

6. In a modulated carrier wave amplifying system, a first amplifierbiased to operate class B, ,a second amplifier biased to pass currentwhen excited by waves of an amplitude less than the means carrier wave,.amplitude,;a thirdamjplifier biased to pass current when excited byseemswaves of an amplitude greater than said: mean carrier waveamplitude, means for "applying modulated carrier wave energy to' saidampli fiers, the phase of the wave energy applied toatleast one or saidamplifiers being displaced 90 relative to the phase of the wave energyappliedto another of said amplifiers, and circuits interconnecting saidamplifiers to a load impedance in a mutual in phase relationship, saidcircuits including networks having inputs and outputs, the impedance atthe inputsof said networks being inversely proportional to the impedanceat the output thereof.

7. In a modulated carrier wave amplifying system, a first amplifierbiased to operate class B, a second amplifier biased to pass currentwhen excited by waves of an amplitude less than the mean carrier waveamplitude, a third and a fourth amplifier biased to pass current whenexcited by waves of an amplitude greater than said mean carrier waveamplitude, means for applying modulated carrier wave energy to all ofsaid amplifiers, the wave energy applied to said second and fourthamplifiers being displaced 90 relative to the wave energy applied tosaid first amplifier and the carrier wave energy ap plied to said thirdamplifier being displaced 180 relative to the phase of the carrier waveapplied to said first amplifier and circuits interconnecting saidamplifiers to a load impedance in a mutual in phase relationship, saidinterconnecting circuits including networks having inputs and outputs,the impedance at the inputs of said networks being inverselyproportional to the impedance at the output thereof.

8. A system for amplifying modulated high frequency oscillations whichis particularly suitable for the final stage of a transmitter and whichcomprises four amplifiers each having an input and an output circuit,the first said amplifier being adjusted to pass current with anyamplitude of alternating voltages applied thereto during one half wave,whereas the other amplifiers are adjusted to pass current only if theamplitude is located above a predetermined threshold value which is forthe second of these amplifiers lower and for the third and fourthamplifiers mutually equal and larger than the carrier wave amplitude, aplurality of networks having input and output terminals and each havingthe property that the input impedance is inversely proportional to theimpedance connected between the output terminals, a load impedanceconnected in series with the outputs of the first and second of saidnetworks and the output of said first amplifier, a series connection ofthe output of said second amplifier and the output of a third networkhaving its input terminals connected to said third amplifier connectedto the input of said first network, the input terminals of said secondnetwork being connected to the fourth amplifier, said oscillations to beamplified being supplied to the inputs of the four amplifiers in suchphase relationship that the voltages supplied by the amplifiers to theload impedance are mutually in phase.

9. A system for amplifying modulated high frequency oscillations whichis particularly suitable for the final stage of a transmitter and whichcomprises four amplifiers each having an input and an output circuit,the first said amplifier being adjusted to pass current with anyamplitude of alternating voltages applied thereto during one half wave,whereas the other amplifiers are adjusted to pass current only if theamplitude is located above a predetermined threshold value which is forthe second of these amplifiers lower and for the third and fourthamplifiers mutually equal and larger than the carrier Wave amplitude, aplurality of networks having input and output terminals and each havingthe property that the input impedance is inversely proportional to theimpedance connected between the output terminals, a load impedanceconnected in series with the outputs of the first and second of saidnetworks and the output of said first amplifier, the surge impedance ofsaid first mentioned network being equal to half the load impedance andhaving connected to its input terminals a series connection of theoutput of said second amplifier and the output of a third network havingits input terminals connected to said third amplifier, the inputterminals of said second network being connected to the fourthamplifier, the surge impedance'of said second and third networks beinghalf that of the first network, said oscillations to be amplified beingsupplied to the inputs of the four amplifiers in such phase relationshipthat the voltages supplied by the amplifiers to the load impedance aremutually in phase.

10. A system for amplifying modulated high frequency oscillations whichis particularly suitable for the final stage of a transmitter and whichcomprises four amplifiers each having an input and an output circuit,the first said amplifier being adjusted to pass current with anyamplitude of alternating voltages applied thereto during one half wave,whereas the other amplifiers are adjusted to pass current only if theamplitude is located above a predetermined threshold value which is forthe second of these amplifiers lower and for the third and fourthamplifiers mutually equal and larger than the carrier wave amplitude, aplurality of networks having input and output terminals and each havingthe property that the input impedance is inversely proportional to theimpedance connected between the output terminals, a load impedanceconnected across the output of one of said networks, the surge impedanceof said load being equal to half that of each of the said networks, aseries connection of the output circuit of the second of said amplifiersand the output of a second of said networks connected in shunt acrosssaid load impedance, the output circuit of the third of said amplifiersbeing connected across the input terminals of said second network, aseries connection of the output of the first of said amplifiers and theoutput of the third of said networks connected to the input of saidfirst network, the output of the fourth of said amplifiers beingconnected to the input of said third network, said oscillations to beamplified being supplied to the inputs of said amplifiers in such phaserelationship that voltages supplied to the load impedance are mutuallyin phase.

11. In combination, a first and second amplifier each having input andoutput circuits, a plurality of impedance inverting networks each havingan input and an output, connections from the output circuits of saidamplifiers to the input and output respectively of one of said networks,each of said connections including the output circuit of another of saidnetworks, a third and a fourth amplifier each having an input circuitand an output circuit, said output circuits being connected to the inputof said last mentioned networks, a load impedance connected for applyingmodulated carrier wave energy to the inputs oi each of said amplifiers.

12. In combination, a first and second amplifier each having input andoutput circuits, a plurality of impedance inverting .networks eachhaving an input and an output, connections from the output circuits ofsaid amplifiers to the input and output respectively of one of saidnetworks, each of said connections including the output circuit ofanother of said networks, a

30,122: to the output of said. one network and means,

third. and afourth. amplifier each having an input circuit: and output.circuit, said output circuits being connected to the input of said lastmentioned networks, a loadimpfidance connected to the output of saidonenetwork and means for applying modulated carrier Wave energy totheinputs of each of said amplifieisin such phase relationship that thevoltages supplied by the amplifiers to theload impedance are mutually inphase 7 KLAAS POSTI-IUMUS.

